This isn't the first time Wario Land music has been in Mario Kart, interestingly. Wario Shipyard from MK7 (which also returns in this game) had a very intense fast-paced remix of the Wario Land leitmotif for its main theme too, but I'm so glad to hear even more Wario Land acknowledgment in Mario Kart here!
I'm really curious where this plays in the game.
What I'm wondering now is if that's MKW's rendition of Wario Shipyard? It's way more mellow than MK7's version, but that made me realize; MKW has day/night and weather effects. Could courses get different music for different conditions? Like maybe this is a sunny daytime rendition.
The original for comparison. The Wario Land theme isn't quite as prominent but it's in there:
My other speculation is that maybe this theme plays in one of the in-between areas leading up to either Wario Shipyard or Wario Stadium, rather than on the track itself, since we know in GP mode you'll have to drive to the next race between each track, necessitating a bunch of little areas on the outskirts of the courses themselves.
Anyway, this is all super cool and I hope it spells more Wario Land representation in the game. (Still holding my breath for a Captain Syrup appearance, or at least a Wario costume alt where he wears his hardhat from WL1 or something)